Scalable effective-temperature reduction for quantum annealers via nested quantum annealing correction
Walter Vinci, Daniel A. Lidar · Physical Review A · 2018
Nested quantum annealing correction (NQAC) is an error-correcting scheme for quantum annealing that allows for the encoding of a logical qubit into an arbitrarily large number of physical qubits. The encoding replaces each logical qubit by a complete graph of degree $C$. The nesting level $C$ represents the distance of the error-correcting code and controls the amount of protection against thermal and control errors. Theoretical mean-field analyses and empirical data obtained with a D-Wave Two quantum annealer (supporting up to 512 qubits) showed that NQAC has the potential to achieve a scalable effective-temperature reduction, ${T}_{\mathrm{eff}}\ensuremath{\sim}{C}^{\ensuremath{-}\ensuremath{\eta}}$, with $0<\ensuremath{\eta}\ensuremath{\le}2$. We confirm that this scaling is preserved when NQAC is tested on a D-Wave 2000Q device (supporting up to 2048 qubits). In addition, we show that NQAC can also be used in sampling problems to lower the effective-temperature of a quantum annealer. Such effective-temperature reduction is relevant for machine-learning applications. Since we demonstrate that NQAC achieves error correction via a reduction of the effective-temperature of the quantum annealing device, our results address the problem of the ``temperature scaling law for quantum annealers,'' which requires the temperature of quantum annealers to be reduced as problems of larger sizes are attempted to be solved.